Earth’s geologic history hidden in quartz grains
Noble gases released from fluid inclusions in quartz minerals tell a story about deep groundwater circulation and rock-forming processes.
Fluids trapped within micro-meter scale inclusions within minerals can be millions to billions of years old and maintain a record of the fluid composition (e.g., halogens, noble gases, CO2,CH4,H2O, H2S, and H2) and environment at the time of mineral genesis. Such mineral fluid inclusions contain information about rock-forming processes through time and deep groundwater circulation.
Noble gases enclosed in mineral fluid inclusions provide important insights about different evolutionary stages of minerals or rock formations and of geological processes, as well as ages of geofluids.
Vapour-rich fluid inclusions in fracture trails in quartz from samples at 918 m (left) and 1319 m (right) depth (Wilske et al., 2022).
A proof-of-concept study was undertaken using rock samples from granites of the Hiltaba Suite in South Australia with core samples collected from 718 – 1935 m from the Blanche 1 geothermal exploration borehole. In very low permeability rock, noble gas isotopes are the only means to evaluate how long fluids were isolated from the water cycle. To extract noble gases from mineral fluid inclusions, a new high vacuum crushing system was developed at the CSIRO noble gas facility.
Experimental set up: A – Sample plate (tungsten carbide, ca 3.5cm Ø), B – Stacked samples plates (4 plates), C – Assembly of single crusher container, D – Pre-vacuum system for crusher container, E -Crusher container (4 pieces) connected to noble gas machine sample ports including hydraulic press (Wilske et al., 2022).
The results of the 20Ne/22Ne and 21Ne/22Ne (nucleogenic) and 40Ar/36Ar (radiogenic) isotope ratios from fluid inclusions in quartz grains isolated from whole granite samples showed a clear indication of an increasing nucleogenic neon (21Ne/22Ne) component with depth; typically, the greater the nucleogenic or radiogenic component, the older the fluid. By comparison, all Australian groundwaters originating from both shallow and deep basins for which noble gases have been measured by CSIRO have had 20Ne/22Ne (~9.8) and 21Ne/22Ne (~0.03) isotope ratios very similar to atmospheric ratios, which are significantly different from those in the fluid inclusions (~7.9 and ~0.06). The increasing nucleogenic contribution with depth is indicative of very old fluid components, supporting the hypothesis that the rock has experienced long-term isolation from the surficial water cycle.